The present invention relates to a loop antenna having a plurality of resonant frequencies and which has particular, but not exclusive, application to reception of signals for Digital Audio Broadcasting (DAB).
Loop antennas are known, for example, from "Antennas" by J. D. Kraus published by McGraw-Hill. One such antenna is shown diagrammatically in FIG. 1 of the accompanying drawing. A wire loop 10 is provided with a pair of feed points 12,14 and a series connected variable capacitor 16, The positions of the feed points 12,14 are generally chosen experimentally to provide a balanced feed to the antenna having an appropriate impedance match, for example 100 .OMEGA.. The resonant frequency of the antenna can be adjusted by altering the value of the capacitor 16. Such an antenna has a fairly good in-band performance but outside of the resonant band performance is generally poor. Accordingly another antenna is usually required to receive or transmit signals at other frequencies which is expensive.